Recent comments in /f/askscience

gravi-tea t1_j29811n wrote

Reply to comment by mutandis57 in has the speed of light always been constant? by 2bornnot2b

Can you help me to understand a few points?

  1. The inflation in the first microseconds of universe that was faster than C refers only to space - not any of the matter or energy?

  2. The expandsion of space that continues is only faster than C when added up over large distances?

  3. Why do these theories assume the universe originating from a single tiny point? Would the math or evidence be much different if it had instead all originated from the size of a neutron or even a golf ball for example?

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VoilaVoilaWashington t1_j295nnw wrote

Reply to comment by Bladestorm04 in How fast does the Milky Way spin? How far does Earth move through space in a year? by Sabre-Tooth-Monkey

CMB isn't any sort of special reference frame. It's just one that can be described as somewhat universal, but in theory, you could use the exact same thing with my neighbour Steve - the rotation of Andromeda is measured relative to Steve. That'll make your math nice and fucky, right?

But then the same is true of the CMB. Imagine if we tried to calculate highway speed limits based on the CMB reference frame. Or even the velocity of Sol relative to the CMB, to pick a larger example - how would that help us calculate how long it takes to get to the other side of the galaxy?

So, you always pick a reference frame that makes your math easy. Speed limits are based on the car's relative speed to the road, ignoring the rotation of the earth and all that. Your ability to juggle on a train is unaffected by the speed of the train relative to the tracks. Earth's rotation around the sun is measured against the sun, not the center of the galaxy.

You can still calculate highway speed limits using Andromeda's approach as a frame of reference, you'll just end up doing a LOT more math depending on where you are on earth and what time of day it is. But you could do it if you really wanted to!

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wabalaba1 t1_j295mhx wrote

Reply to How much does the liquid magma of the Earth affect it's surface temperature? by tripperfunster

So there's an interesting story here, but we need to go on a bit of a long walk involving lava lamps, a magnetic-field paradox, and iron snow, to see the whole picture.

Currently, we believe that almost all of the planet was molten liquid when the Earth finished forming, due to the intense heat from all the billions of collisions that make a planet. Since then, Earth has been slowly cooling by conducting its heat up through the mantle and then radiating that heat out into space. Because space is cold and the centre of Earth is hot, we might expect that the Earth would "freeze" (solidify) from the outside inward.

More rock means better insulation, and so a larger planet can insulate its liquid core better. This is why small bodies like the Moon (and Mars?) don't still have a magnetic field--their cores cooled quicker due to having less insulating rock atop their cores. Earth is the biggest rocky planet around the Sun, and has a very well-insulated core.

But there's a big problem with this, and it has to do with the question of exactly how insulating is that rock? The classic theory about our magnetic field is that there is convection in our liquid outer core. Convection is what makes a lava lamp: the hot lightbulb at the bottom of the lamp heats the wax, and as the wax heats up it expands, and as it expands it becomes less dense than the water, and so it floats to the top like a lifejacket. Colder wax at the top contracts, becomes more dense than water, and sinks.

If you somehow had a liquid-iron lava lamp, this convection would also create electrical currents and a magnetic field (because physics, I dunno). Our liquid-iron outer core has long been thought to work like an iron lava lamp: the deepest part of the outer core gets heated by the (VERY HOT) inner core. That makes the liquid iron expand and rise up. At the top, it conducts heat away into the mantle. The liquid metal now contracts a bit and sinks again. These circular currents create the Earth's powerful magnetic field.

Except, they don't. A few studies about a decade ago (ish) started showing that the mantle (which is solid rock) conducts heat too well. Heat is leaving the core faster than we thought. Scientists know how old the Earth is, so, when they compared the measured rate of heat leaving against the age of the Earth, they discovered something shocking: the core is now too cold to run the heat engine to make our magnetic field!

This would be apocalyptic, except for the fact that we very much do still have a magnetic field. It doesn't make sense. So how do we explain that? Something else clearly must be making the liquid iron circulate.

The answer to this lies in the answer to a question I've had since grade school: WHY is there a solid inner core and a liquid outer core?

Remember: scientists have long agreed that probably almost all of Earth was liquid at the start. The inner core didn't always exist. As the convection of liquid iron carries heat out to the mantle, the whole core itself is cooling over geologic time. Eventually, some of that liquid gets cold enough to crystallize. Particles of solid iron grow and (being denser than liquid) sink as flakes of iron snow downward. This slowly grows to form an inner core of the Earth!

Finally: even if we don't have enough heat to drive circulation in the outer core, we can drive it by this crystallization. This is current, ongoing research by people smarter than me, so the details may vary. But the heart of the matter is that downward-falling "snow" will displace hot fluid on the surface of the inner core, which is forced to rise. So, if what I just wrote made sense, then it is now the process that's growing our inner core that is powering the currents which make Earth's magnetic field--which protects us from solar radiation.

Which has all been a very long way around to say that the temperature/physical state (solid vs. liquid) of the core does indeed affect conditions on the surface.

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CrustalTrudger t1_j2938m6 wrote

Reply to thermal expansion affecting climate change? by mjoninha

> What is the source of energy that raises the level of sea water through thermal expansion?

The increase in the average temperature of the atmosphere. In short, the ocean absorbs heat from the atmosphere, so if the atmosphere warms up, the ocean will warm up as well. Accounting for the behavior of the ocean as effectively a giant heatsink has been shown to be important for understanding temporal changes in atmospheric temperature (e.g., Kosaka & Xie, 2013).

> Is thermal expansion global?

Yes, but it's not necessarily uniform. We can break out two primary components of sea level rise, mass addition (i.e., melting glaciers and continental ice sheets adding mass to the ocean) and so-called steric components that are changes in volume related to changes in density. Within the steric components, both the salinity and temperature influence this, i.e., colder and saltier water is denser and thus for a given mass takes up less volume. Spatial (and temporal) variations in both temperature and salinity mean that the steric component of sea level rise varies spatially, for example, a relatively high rate of sea level rise (compared to global averages) along the central east coast of the US has been partially attributed to warmer, fresher waters in these regions (e.g., Sallenger et al., 2012).

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majorpickle01 t1_j292333 wrote

Reply to has the speed of light always been constant? by 2bornnot2b

In short, yes.

In not short, there are some more fringe theories coming out of quantum mechanics that posit that the speed of light (or more specifically the speed of causality) has changed slightly over time. However I've never seen anything serious come out of such papers.

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